This invention relates to a method for the recovery of platinum. More particularly, the present invention relates to a method for recovering platinum from spent catalysts.
Among the more prominent industries employing the catalysis characteristics of platinum is the petrochemical industry. Thus, platinum catalysts, typically deposited on a carrier such as alumina and the like, are widely used in petrochemical applications for hydrogenation, dehydrogenation, cracking, refining, detoxification of waste gases, etc.
It has been found that with each succeeding application, the platinum catalysts evidence a reduction in efficiency, such being attributed either to the accumulation of sediments formed by secondary reactions during the processing sequence or to the presence of deleterious materials entering the reaction system with raw materials. Heretofore, it has been common practice to effect regeneration of the catalysts by annealing at elevated temperatures to remove organic materials, by saturating the catalyst with an ammonium nitrate or chloride solution and finally heating to a temperature ranging from 400.degree.- 500.degree. C. Nonetheless, studies have revealed that catalysts regenerated in this fashion several times lose their effectiveness. At that juncture, it is advantageous to recover the precious metal and this end is normally attained by a processing procedure categorized either as a chlorination or leaching method.
In the chlorination procedure, the spent catalyst is roasted in air at temperatures ranging up to 870.degree. C and exposed at such temperatures to chlorinating agents such as aluminum chloride, silicon tetrachloride, carbonyl chloride and the like. The resultant platinum compounds are absorbed by water and reclamation of platinum therefrom is effected from solution by reduction with aluminum, zinc and the like.
The leaching technique may be based either upon an acid or alkaline reaction. The acid process also involves roasting the catalyst at a temperature in excess of 800.degree. C to remove organic substances and, subsequently, leaching for several hours with a 20 - 60% sulfuric acid solution or a 20 - 50% hydrochloric acid solution at temperatures ranging from 100.degree. - 140.degree. C. Oftimes, this reaction is effected at superatmospheric pressures ranging up to 5.0 MPa (50 kp/cm.sup.2). Platinum may then be recovered from a separated sludge.
In the alkaline process, the spent catalyst, either annealed or non-annealed, is melted with an excess of sodium hydroxide, the resultant melt being leached with water and the platinum concentrate converted to platinum. Dissolution of non-annealed catalysts in such processes may typically be attained by means of a 40 percent sodium hydroxide solution at temperatures ranging from 120.degree. - 145.degree. C.
Unfortunately, each of these prior art processes has inherent limitations. The platinum catalysts regenerated in the foregoing manner are freed from organic compounds at temperatures ranging from 800.degree. - 1200.degree. C, thereby increasing power costs and introducing complexities in processing. Additionally, the plurality of steps in the regeneration process, namely, roasting, melting, sedimentation, decantation, filtering, etc., results in platinum losses ranging from 3 - 5 percent.
Similarly, the alkali and acid processes are limited by aluminate liquor processing and high acid volume requirements, respectively. Accordingly, efforts to overcome these difficulties have continued.